Photoconductive recording film of the type useful for recording motion pictures is made by combining a plastic film substrate of the same type as used for motion picture film but with a photoconductive layer rather than a silver-halide layer. During the formation of the plastic film strip, a crystalline photosensitive substance such as cadmium sulfide is deposited upon the plastic strip in a continuous layer. When the film is exposed in an appropriate manner to an optical image, the photosensitive layer upon the film accumulates an electrostatic charge proportional to the flux of incident light from the image. Specifically, when recording an image, the accumulated charge determines the gray level of the image. This may be compared to normal film where silver grains interact with light to form silver-halides and their density determines the gray level of the image. The density or grain size of the cadmium sulfide on photoconductive film is related to its resolution rather than the gray level. The resolution of the image recorded on the photoconductive film is limited only by two factors: the imaging of the film and the recovery of the image. The limit to the imaging is dominated by the grain size of the cadmium sulfide or photosensitive crystal layer upon the film, and the resolution of the optics used to focus the image onto the film. The recovery of the image is limited by the size of the sensor used to measure the charge density over the film. One example of photoconductive film which is commercially available is Kodak Photoconductive Recording Film, SO-101 which may be purchased from Kodak Film Company, Rochester, New York.
After an image is recorded onto the photoconductive film strip, the image may be read out by passing the film over a device which senses the electrostatic charge accumulated in the photosensitive layer. For example, a plurality of microscopic charge sensing electrodes arranged in a linear array extending across the width of the film may be used to measure the charge density in the line form as the film is moved longitudinally past this linear array of electrodes. In a like manner, charge sensing electrodes could be arranged in a 2-dimensional pattern in such a manner that the image could be recovered at a later time by appropriate juxtaposition of the various measured values at each point over the array. The charge sensing electrodes can be termed "floating" because their electrical potentials are permitted to float so as to follow the amount of static charge in the underlying film. The change in the floating potential of each one of the electrodes represents the static charge accumulated in the photosensitive layer of the film in the region beneath the floating electrode. Thus, each floating electrode corresponds to an individual area element or pixel, each frame or image recorded on the film being divided into a plurality of such pixels. The voltage at each one of the floating electrodes is sensed in an output electronic device which generates a video signal therefrom.
Several difficulties exist with such a read out device. First, the production of such a line or area array of microscopic sense electrodes with proper and constant geometrical positioning is extremely difficult to achieve. Secondly, the distance between the film surface and each sense electrode within the array of floating electrodes is difficult to control. The amplitude of the signal generated by each of the floating electrodes varies as the reciprocal of the distance between the film and the floating electrode. Therefore, the inability to control the film-to-floating electrode distance reduces the ability to accurately measure the charge at each pixel.
Another difficulty with the planar array read out device is that the floating electrodes must be periodically reset to a known potential in order to minimize the effect of unavoidable change of drift in the voltage of each of the floating electrodes. Even in the absence of the photoconductive film, the voltage of some of the floating electrodes may drift thereby yielding errors in the recovered image. The problem may be avoided by prerecording reset strips across the width of the film at periodic intervals along the length thereof, the reset strips each having a predetermined amount of static charge density. The linear space or distance between the reset strips is determined by the electronic time constant associated with the decay or drift of the sensing electrodes and the speed at which the film is moved past the electrode array. An electronic circuit resets the potential of each of the floating electrodes just as each reset strip passes beneath them. The image is recorded between adjacent reset strips on the film. The disadvantage of this method for resetting the floating electrodes is that each reset strip occupies a region of the film which would otherwise be occupied by the image, so that this loss significantly reduces the amount of information which may be recorded on a given length of photoconductive film and, furthermore, periodically interrupts the information sensed by the charge sensing electrodes.
The purpose of the present invention is to solve the problems of uncontrolled variations in film-to-readout sensor array distance, the interruption of recorded film information due to the presence of the periodic reset strips along the length of the film, and the elimination of the difficult geometrical arrangement for the sense electrodes on a readout array.